Fuel cells are a kind of power generator which extracts electric energy through electrochemical oxidation of fuels such as hydrogen and methanol. In recent years, the fuel cells have drawn attention as a clean energy supply source. Among fuel cells, polymer electrolyte fuel cell is operated at a low standard working temperature of approximately 100° C., and provides high energy density, and thus the polymer electrolyte fuel cell is expected to be widely applied as relatively small-scale distributed power facilities and as mobile power generator on automobile, ship, and the like. In addition, the polymer electrolyte fuel cell also draws attention as power source of small-scale mobile apparatus and portable apparatus, and is expected to be mounted on cell phone, personal computer, and the like in place of nickel-hydrogen battery and lithium-ion battery.
A normal fuel cell is constituted by cell units, the cell unit having a configuration of a membrane electrode assembly (hereinafter referred to also as MEA) being sandwiched between separators, which MEA is constituted by an anode electrode and a cathode electrode in which a reaction of power generation occurs, and by a polymer electrolyte membrane serving as a proton conductor between the anode and the cathode. The polymer electrolyte membrane is mainly constituted by a polymer electrolyte material. The polymer electrolyte material is also used as a binder of an electrode catalyst layer and the like. The characteristics required of the polymer electrolyte membrane include, first, high proton conductivity, specifically high proton conductivity under high temperature and low-humidification conditions. Furthermore, since the polymer electrolyte membrane also functions as a barrier that prevents direct reaction between fuel and oxygen, low permeability of fuel is required. Other necessary characteristics include chemical stability for withstanding strong oxidizing atmosphere during operation of the fuel cell, mechanical strength and physical durability of withstanding thinning of membrane and repeated swelling-drying cycles, and the like.
Conventionally, as the polymer electrolyte membranes, there is widely used Nafion (registered trade name, manufactured by DuPont) which is a perfluoro-sulfonate-based polymer. Since Nafion (registered trade name) is manufactured through multistage of synthesis, it has problems of being extremely expensive and of large fuel-crossover (transmission amount of fuel). In addition, as to Nafion, there were pointed out a problem of losing membrane mechanical strength and physical durability by swelling-drying, a problem in which the use at high temperatures is not possible because of low softening point, a problem of waste disposal after use, and further an issue of difficulty in recycling the material.
To the situation, the development of hydrocarbon-based electrolyte membranes has been actively conducted in recent years as a polymer electrolyte material having excellent membrane characteristics at a low price substituting Nafion (registered trade name).
For example, there is provided a block copolymer having a segment not introducing sulfonic acid group and a segment introducing sulfonic acid group therein, wherein the former segment includes a polyethersulfone (PES), and the latter segment includes a sulfonated polyethersulfone, (Patent Documents 1, 2, and 3). These block copolymers, however, use an amorphous polymer having high glass transition temperature as the base skeleton, thus raising a problem of brittleness and poor physical durability, and further giving poor hot water resistance and poor physical durability because of containing a large number of sulfone group having high water absorbency.
Patent Document 3 describes a block copolymer in which the former segment is constituted by a tough polyether ketone (PEK), and the latter segment is constituted by a sulfonated polyether ether ketone. The description, however, gives the PEK only as an example of preferred structure, and does not give the synthesis of PEK segment, which does not introduce sulfonic acid group, of crystalline and insoluble in solvent. Therefore, the description is not a detailed investigation of the block copolymer. Furthermore, since the PEK contains phenylene group and biphenylene group, having high electron density and high reactivity, being sandwiched between ether groups, and since the PEK introduces sulfonic acid group into the activated above groups, there gives insufficient chemical stability to oxidation deterioration, desulfonation, and the like.
As described above, the polymer electrolyte materials according to related art are insufficient as means for improving economic efficiency, processability, proton conductivity, mechanical strength, chemical stability, and physical durability, thus they cannot serve as industrially useful polymer electrolyte materials.